Method for measuring an angularly resolved intensity distribution and projection exposure apparatus

ABSTRACT

A method for measuring an angularly resolved intensity distribution in a reticle plane ( 24 ) of a projection exposure apparatus ( 10 ). The apparatus includes an illumination system ( 16 ), irradiating a reticle ( 22 ) arranged in the reticle plane ( 24 ) and having a first pupil plane ( 20 ). All planes of the projection exposure apparatus which are conjugate thereto are further pupil planes, and the reticle plane ( 24 ) and all planes which are conjugate thereto are field planes. The method includes: arranging a spatially resolving detection module ( 44 ) in the region of one of the field planes ( 24, 30 ) such that the detection module is at a smaller distance from this field plane than from the closest pupil plane ( 20 ), radiating electromagnetic radiation ( 21 ) onto an optical module ( 42 ) from the illumination system, and determining an angularly resolved intensity distribution of the radiation from a signal recorded by the detection module.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.14/796,328, filed Jul. 10, 2015, which is a continuation of U.S. patentapplication Ser. No. 13/935,972, filed Jul. 5, 2013, now U.S. Pat. No.9,081,294, which claims benefit under 35 U.S.C. § 119(e) to U.S.Provisional Application No. 61/668,554, filed Jul. 6, 2012, and whichclaims priority under 35 U.S.C. § 119(a) to German Patent ApplicationNo. 10 2012 211 846.2, also filed on Jul. 6, 2012. The disclosures ofthese related applications are hereby incorporated into the presentapplication by reference in their respective entireties.

FIELD OF AND BACKGROUND OF THE INVENTION

The invention relates to a method for measuring an angularly resolvedintensity distribution in a reticle plane of a projection exposureapparatus for microlithography, and to a projection exposure apparatuscomprising a measurement system for measuring an angularly resolvedintensity distribution. Furthermore, the invention relates to a methodfor measuring a beam divergence in an illumination system of aprojection exposure apparatus for microlithography, and to a projectionexposure apparatus.

Lenses of projection exposure apparatuses for microlithography are beingoperated with ever smaller k1 factors. This has the consequence that aprecise measurement and specification of the illumination system of suchprojection exposure apparatuses is becoming more and more important.Such an illumination system irradiates a reticle to be imaged with apreviously selected angular distribution. The angular distributionchosen is also designated as an illumination setting. Examples of suchillumination settings include annular illumination, dipole illuminationand quadrupole illumination. In particular, it is important to be ableto characterize the illumination setting even when the projection lensand the illumination system are integrated in a projection exposureapparatus.

The illumination setting in a projection exposure apparatus isconventionally characterized by measuring a so-called “pupilogram” witha sensor integrated in the wafer stage of the projection exposureapparatus. A definition of a pupilogram is included e.g. in the articleby Joe Kirk and Christopher Progler “Pinholes and pupil fills”,Microlithography World, Autumn 1997, pages 25 to 34. In order togenerate the pupilogram, a special measurement reticle with pinholestructures arranged thereon is loaded into the reticle plane of theprojection exposure apparatus. The above-mentioned sensor is generallyarranged for measurement purposes below the wafer plane, to be precisein a conjugate pupil plane of the projection lens. This has the effectthat the intensity distribution which is present in the pupil plane andwhich corresponds to the angularly resolved intensity distribution inthe reticle plane is imaged on the sensor.

If this method is used during the projection operation of the projectionexposure apparatus, then generally a relatively significant time delayarises since firstly the production reticle has to be removed from thewafer stage and the pinhole reticle has to be loaded for carrying outthe measurement. Furthermore, in the case of generally older projectionexposure apparatuses, the problem often arises that such apparatuses arenot provided with a suitable sensor arranged below the wafer plane forcarrying out the pupilogram measuremen.

OBJECTS AND SUMMARY OF THE INVENTION

It is an object of the invention to solve the problems mentioned aboveand, in particular, to provide a measuring method and a projectionexposure apparatus whereby it is possible to measure the angularlyresolved intensity distribution in the reticle plane during theprojection operation of the projection exposure apparatus withoutthereby creating a long time delay.

The above object is achieved according to one formulation of theinvention for example by the method mentioned below for measuring anangularly resolved intensity distribution in a reticle plane of aprojection exposure apparatus for microlithography. The projectionexposure apparatus comprises an illumination system, which is configuredfor irradiating a reticle arranged in the reticle plane and has a firstpupil plane. All planes of the projection exposure apparatus which areconjugate with respect to the first pupil plane are further pupilplanes. The reticle plane and all planes which are conjugate withrespect thereto are field planes. The method according to thisformulation of the invention comprises: arranging an optical module inthe beam path of the projection exposure apparatus, and arranging aspatially resolving detection module in the region of one of the fieldplanes in such a way that the detection module is at a smaller distancefrom this field plane than from the closest pupil plane. Furthermore,the method comprises: radiating electromagnetic radiation onto theoptical module with the illumination system, and determining anangularly resolved intensity distribution of the radiated radiation froma signal recorded by the detection module.

In other words, according to this formulation of the invention, theangularly resolved intensity distribution in the reticle plane ismeasured with an optical module and a detection module. The detectionmodule is disposed downstream of the optical module in the beam path ofthe projection exposure apparatus. The detection module is arranged inthe region of one of the field planes of the projection exposureapparatus, for example in the reticle plane or the wafer plane. In thiscase, an arrangement in the region of one of the field planes isunderstood to mean that the distance between the detection module, inparticular a detection area of the detection module, and the relevantfield plane is smaller than the distance between the detection moduleand the closest pupil plane. In particular, the distance between thedetection module and the relevant field plane is a maximum of half, inparticular a maximum of one quarter, of the distance between thedetection module and the closest pupil plane. The detection module istherefore situated substantially in the near field of the correspondingfield plane.

In accordance with the method according to this formulation of theinvention, for example, the optical module together with the detectionmodule can be inserted above the production reticle loaded from thereticle stage. It is thus possible to carry out the illumination settingduring production operation, e.g. between the exposure of two wafers,without further loss of time. Moreover, according to the methodaccording to the invention it is not necessary to provide a sensorarranged below the wafer plane in the wafer displacement stage, alsocalled wafer stage. Therefore, the method according to the invention canbe carried out, in particular, also in the case of older projectionexposure apparatuses which do not have a corresponding sensorincorporated in their wafer displacement stage.

In accordance with one embodiment according to the invention, theoptical module is arranged in the region of one of the field planes insuch a way that the optical module, in particular the upper edge of theoptical module that is irradiated during the measurement, is at asmaller distance from this field plane than from the closest pupilplane. In particular, the optical module is arranged in the region ofone of the field planes in such a way that the optical module is at adistance of a maximum of half, in particular a maximum of one quarter,of the distance between the optical module and the closest pupil plane.

In accordance with a further embodiment according to the invention, theoptical module of the detection module is arranged in the region of thesame field plane. The optical module and the detection module can thusbe integrated into a measurement system with a compact construction.

In accordance with a further embodiment according to the invention, theoptical module is arranged in the region of the reticle plane during themeasurement. This makes it possible to measure the angularly resolvedintensity distribution particularly precisely, since no corruption canoccur for instance as a result of imaging aberrations of interposedoptical elements. In accordance with a further embodiment according tothe invention, the detection module is likewise arranged in the regionof the reticle plane.

In accordance with a further embodiment according to the invention, theoptical module comprises two diffraction gratings arranged successivelyin the beam path of the incoming radiation. Preferably, the measurementsystem comprising the optical module and the detection module isdesigned as a shearing interferometer. In accordance with oneembodiment, the relative situation and thus the relative position and/orthe relative orientation of the phase gratings with respect to oneanother is varied during the measurement of the intensity distribution.

In accordance with a further embodiment according to the invention, aspatial coherence function is recorded by the detection module and thespatial coherence function is thereupon converted into the angularlyresolved intensity distribution in the reticle plane. The conversion canbe effected using the van cittert-zernike theorem.

In accordance with a further embodiment according to the invention, theoptical module comprises a focusing optical element, with which theradiated radiation is focused onto the detection module. The detectionmodule is thus arranged in the region of a focal plane of the focusingoptical element.

In accordance with a further embodiment, already discussed above,according to the invention, for the purpose of measuring the angularlyresolved intensity distribution, a measurement system comprising theoptical module and the detection module is inserted into the beam pathof the radiated radiation above a reticle arranged in the reticle plane.In the case of this variant, the angularly resolved intensitydistribution can be measured with a product reticle arranged in thereticle plane.

In accordance with a further embodiment according to the invention, theoptical module is fixed to an edge region of a reticle displacementstage of the projection exposure apparatus, in particular integratedinto the edge region. In this context, an edge region should beunderstood to mean a region of the reticle which, with the reticleloaded, is not concealed by the latter.

In accordance with a further embodiment according to the invention, theangularly resolved intensity distribution is measured for a plurality offield points in the reticle plane simultaneously.

In accordance with a further embodiment according to the invention, thedetection module comprises an integral spatially resolving area sensor.An appropriate spatially resolving area sensor is a CCD sensor, forexample.

In accordance with a further embodiment according to the invention, thedetection module comprises at least two point sensors separated from oneanother. By way of example, a four-quadrant diode can be used.

In accordance with a further embodiment according to the invention, theoptical module comprises a shadow casting element, which is at leastpartly non-transmissive to the radiated radiation, such that at leastone shaded region is generated on a detection module. For the purpose ofdetermining the angularly resolved intensity distribution, the positionof a transition from the shaded region to an unshaded region adjacentthereto is determined using the detection module. In accordance with onevariant, the shadow casting element is designed as a grating.Furthermore, the detection area of the detection module can be providedwith measurement markings in the form of a further grating.

In accordance with a further embodiment according to the invention, theoptical module has a pole selection device, which comprises a pinholestop and a blocking element for blocking part of the radiated radiation,said blocking element being offset with respect to said pinhole stop ina direction transversely with respect to the reticle plane. The blockingelement can be configured e.g. as a stop.

The abovementioned object can be achieved according to a furtherformulation of the invention with a projection exposure apparatus formicrolithography. The projection exposure apparatus according to theinvention comprises an illumination system, which is configured forirradiating a reticle arranged in a reticle plane of the projectionexposure apparatus with electromagnetic radiation. The illuminationsystem has a first pupil plane, wherein all planes which are conjugatewith respect to the first pupil plane are further pupil planes of theillumination system. The reticle plane and all planes which areconjugate with respect thereto are field planes. The projection exposureapparatus according to the invention furthermore comprises a measurementsystem, which has an optical module and a detection module and isconfigured for measuring an angularly resolved intensity distribution ofthe radiated radiation in the reticle plane. Furthermore, the projectionexposure apparatus comprises a positioning device for arranging thedetection module in the region of one of the field planes in such a waythat the detection module is at a smaller distance from this field planethan from the closest pupil plane. The optical module is configuredaccording to the invention for generating a radiation distribution onthe detection module arranged in the region of the field plane, fromwhich radiation distribution it is possible to determine the angularlyresolved intensity distribution in the reticle plane.

The explanations and features given above with regard to the methodaccording to the invention can be applied to the projection exposureapparatus according to the invention.

In the development of lithography processes for the production ofsemiconductor chips, taking account of diffraction effects that occurduring the imaging of the mask structures onto the wafer is often ofgreat importance. Therefore, the mask structures on the productionreticles used are correspodningly modified for the correction of saiddiffraction effects. This correction is generally designated as opticalproximity correction (OPC). The optimization of the optical proximitycorrection conventionally often takes place on a reference exposureapparatus. However, the masks optimized via the reference exposureapparatus are subsequently used on a plurality of projection exposureapparatus. Therefore, there is a need to perform optical proximitycorrections separately in a tailored manner with regard to eachindividual exposure apparatus. One parameter which significantlyinfluences the optical proximity correction is the beam divergence ofthe exposure radiation in the input region of the illumination system ofthe projection exposure apparatus. Such an illumination system has abeam expanding optical unit for expanding the beam cross section of theexposure radiation emerging from a radiation source. The divergence ofthe beam directly downstream of the beam expanding optical unit has aconsiderable influence on the optical proximity correction.

In principle, it is conceivable to measure the beam divergence directlyat the output of the beam expanding optical unit e.g. using suitablyadapted sensors according to the Shack-Hartmann principle. However, sucha measurement requires a service action at the projection exposureapparatus, which results in a production stoppage of the apparatus.Therefore, it is an aim to measure the beam divergence in an inputregion of the illumination system of a projection exposure apparatuswith a high accuracy, without having to interrupt the operation of theexposure apparatus for a long time.

For this purpose, in accordance with a further aspect of the invention,a method for measuring a beam divergence in an illumination system of aprojection exposure apparatus, for microlithography is provided, whereinthe illumination system has a beam expanding optical unit for expandingthe beam cross section of an exposure radiation emerging from aradiation source, and also a beam angle redistribution module fordeflecting partial beams of the exposure radiation emerging from thebeam expanding optical unit. The method according to this aspect of theinvention comprises: exchanging at least one element of the beam angleredistribution module for a divergence amplification module, which isconfigured for amplifying the divergence of the beam emerging from thebeam expanding optical unit, and measuring the illumination angledistribution in the reticle plane and determining the divergence of theexposure radiation emerging from the beam expanding optical unit fromthe measured illumination angle distribution.

Depending on the embodiment, the beam angle redistribution module can beformed by a diffractive optical element or else comprise a plurality ofoptical elements. As a result of the exchange according to the inventionof an element of the beam angle redistribution module, in particular ofthe diffractive optical element, for a divergence amplification module,the divergence in an input section of the illumination system, inparticular directly after the beam expanding optical unit, is increasedin a predetermined manner.

As mentioned above, for example for optimizing the optical proximitycorrection of lithography masks there is great interest in accuratelyknowing the divergence in said input section of the illumination system,also designated hereinafter as input divergence. The divergenceamplification according to the invention makes it possible to determinethe input divergence with high accuracy using an angularly resolvedmeasurement of the intensity distribution at a position disposeddownstream of the illumination system in the beam path of the projectionexposure apparatus.

Thus, by way of example, the measurement can be effected with thepupilogram measurement already mentioned above, which is known from theprior art and already provided in many projection exposure apparatuses.In accordance with one embodiment, for this purpose, a pinhole reticleis loaded into the reticle plane and the pupilogram generated isrecorded by a sensor integrated in the wafer stage. The divergence atthe location of the divergence amplification module is calculated fromthe recorded pupilogram. From this value in turn, the divergenceamplification is calculated and the divergence in the input section ofthe illumination system during normal operation is thereby determined.

The divergence amplification according to the invention increases theeffective accuracy of the measurement of the input divergence calculatedback. Without the divergence amplification according to the inventionfor the input divergence, the pupilogram measuring method would onlyyield measurement results with low accuracy. The accuracy achievable inthis case would not suffice for instance for optimizing the proximitycorrection of the reticle structures.

The invention thus opens up the possibility of using measuring systemsalready present in the projection exposure apparatus for the divergencemeasurement and thereby determining the required divergence measurementvalues in a cost-effective manner. Furthermore, it is also possible tocarry out the input divergence measurement using a separate measurementsystem arranged in the region of the reticle plane, for example. Inaccordance with one embodiment according to the invention, for thispurpose the measurement system is used in one of the variants accordingto the invention, that is to say that the illumination angledistribution in the reticle plane is measured using the measurementsystem described above.

In accordance with one embodiment—already discussed above—of the methodaccording to the further aspect of the invention, the projectionexposure apparatus has a projection lens for imaging mask structuresfrom the reticle plane onto a wafer, and a wafer stage for holding thewafer, and the illumination angle distribution is measured with adetector module arranged at the wafer stage.

Furthermore, in accordance with the further aspect of the invention, aprojection exposure apparatus for microlithography comprising aradiation source for generating exposure radiation, and an illuminationsystem disposed downstream of the radiation source and serving forradiating the exposure radiation into a reticle plane of the projectionexposure apparatus, is provided. The illumination system according tothe invention comprises: a beam expanding optical unit for expanding thebeam cross section of the exposure radiation emerging from the radiationsource, a beam angle redistribution module for deflecting partial beamsof the exposure radiation emerging from the beam expanding optical unit,a divergence amplification module for amplifying the divergence of thebeam emerging from the beam expanding optical unit, and an exchangingdevice for exchanging at least one element of the beam angleredistribution module for the divergence amplification module.

In accordance with one embodiment, the projection exposure apparatus inaccordance with the further aspect according to the invention has aprojection lens for imaging mask structures from the reticle plane ontoa wafer, a wafer stage for holding the wafer, and a system for measuringthe illumination angle distribution in the reticle plane. The systemcomprises a detector module arranged at the wafer stage, said detectormodule being arranged in the region of the conjugate pupil plane of theprojection lens during the measurement process. Preferably, for carryingout the measurement a measurement mask having punctiform structures isarranged in the reticle plane.

In a further embodiment—already discussed above—in accordance with thefurther aspect according to the invention, the beam angle redistributionmodule comprises a diffractive optical element. The exchanging devicethus serves to replace the diffractive optical element by the divergenceamplification module.

In a further embodiment in accordance with the further aspect accordingto the invention, the divergence amplification module comprises amicrolens element array. In particular, the divergence amplificationmodule is designed as a microstructured plate in which the microlenselement array is formed by the microstructuring.

In a further embodiment in accordance with the further aspect accordingto the invention, the divergence amplification module comprises twomicrolens element arrays having different focal lengths. Preferably, themicrolens element arrays are spaced apart from one another in such a waythat the divergence amplification module acts as a telescope opticalunit.

With regard to the further aspect of the invention, too, it holds truethat the features specified with regard to the abovementionedembodiments of the method according to the invention can correspondinglybe applied to the projection exposure apparatus according to theinvention. Conversely, the features specified with regard to theabovementioned embodiments of the projection exposure apparatusaccording to the invention can correspondingly be applied to the methodaccording to the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments according to the invention are explained ingreater detail below with reference to the accompanying schematicdrawings, in which:

FIG. 1 shows a schematic illustration of an embodiment according to theinvention of a projection exposure apparatus for microlithographycomprising a system for measuring an angularly resolved intensitydistribution in a reticle plane of the projection exposure apparatus,

FIG. 2 shows a further embodiment according to the invention of aprojection exposure apparatus comprising a measurement system of theabovementioned type,

FIG. 3 shows a further embodiment according to the invention of aprojection exposure apparatus comprising a measurement system of theabovementioned type,

FIG. 4 shows a schematic sectional view of an embodiment according tothe invention of an optical module of the measurement system inaccordance with one of FIGS. 1 to 3,

FIG. 5 shows a schematic sectional view of the measurement system in afurther embodiment according to the invention,

FIG. 6 shows a schematic sectional view of the measurement system in afurther embodiment according to the invention,

FIG. 7 shows a plan view of a detection module of the measurement systemin an embodiment according to the invention,

FIG. 8 shows a schematic sectional view of the measurement system in afurther embodiment according to the invention,

FIG. 9 shows a schematic sectional view of a previously knownmultimirror array that is optionally employed in the illumination systemof a projection exposure apparatus,

FIG. 10 shows a schematic sectional view of an illumination system of aprojection exposure apparatus for microlithography comprising anexchanging device for the replacement according to the invention of adiffractive optical element of the illumination system by amicrostructured plate,

FIG. 11 shows a schematic sectional view of the microstructured plate inaccordance with FIG. 10, and

FIG. 12 shows a schematic sectional view of a projection exposureapparatus in a measuring configuration for measuring an angularlyresolved intensity distribution in the reticle plane of the projectionexposure apparatus.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS ACCORDING TO THE INVENTION

In the exemplary embodiments described below, elements that arefunctionally or structurally similar to one another are as far aspossible provided with the same or similar reference signs. Therefore,for understanding the features of the individual elements of a specificexemplary embodiment, reference should be made to the description ofother exemplary embodiments or the general description of the invention.

In order to facilitate the description, a Cartesian xyz coordinatesystem is indicated in the drawing and reveals the respective positionalrelationship of the components illustrated in the figures. In FIG. 1,y-direction runs perpendicularly to the plane of the drawing out of thelatter, the x-direction runs toward the right, and the z-direction runsupward.

FIG. 1 schematically illustrates a projection exposure apparatus 10 formicrolithography in a first embodiment according to the invention. Theprojection exposure apparatus 10 comprises, as is customary in the caseof projection exposure apparatuses known from the prior art, a radiationsource 12 for generating electromagnetic radiation 14 having awavelength suitable for lithography of e.g. 365 nm, 248 nm or 193 nm.The projection exposure apparatus 10 can also be designed for radiationin the extreme ultraviolet wavelength range (EUV). In this case, theelectromagnetic radiation 14 has a wavelength of <100 nm, e.g.approximately 13.5 nm.

An illumination system 16 is disposed downstream of the radiation source12 and is configured for radiating the radiated electromagneticradiation 14 with a predefined angular distribution onto a reticle 22with mask structures arranged thereon. The predefined angularly resolvedintensity distribution generated by the illumination system 16 isdefined by the so-called illumination setting. Examples of illuminationsettings include annular illumination, quadrupole illumination or other,more complex illumination configurations.

The illumination system 16 has an optical axis 18 and generallycomprises a plurality of optical elements. A pupil plane 20 is arrangedwithin the illumination system 16, in which pupil plane the radiationdistribution is Fourier-transformed relative to the reticle plane 24 ofthe projection exposure apparatus 10. The reticle 22 is held in adisplaceable manner by a reticle displacement stage 25, also designatedas “reticle stage”, in the reticle plane 24.

The projection exposure apparatus 10 furthermore comprises a projectionlens 26 for imaging the mask structures of the reticle 22 from thereticle plane 24 into a wafer plane 30. During an imaging process, awafer 28 is arranged on a wafer displacement stage 32, also designatedas “wafer stage”, in the wafer plane 30.

According to the invention, the projection exposure apparatus 10comprises a measurement system 40 for measuring the angularly resolvedintensity distribution of the illumination radiation 21 radiated intothe reticle plane 24. The measurement system 40 comprises an opticalmodule 42 and a detection module 44. In the embodiment shown, theoptical module 42 is arranged in the region of the reticle plane 24, tobe precise in an edge region of the reticle displacement stage 25 thatis provided therefor. The detection module 44 is situated directly belowthe optical module 42.

In accordance with an embodiment illustrated in FIG. 4, the opticalmodule 42 comprises two diffraction gratings 46 and 48 arrangedsuccessively in the beam path, as shown in FIG. 4. The incomingillumination radiation 21 is split by the first diffraction grating 46into two partial beams 50 and 52 running obliquely with respect to oneanother. The two partial beams are pivoted by the second diffractiongrating 48, such that they run parallel to one another again, asillustrated by the reference signs 50-1 and 52-1 in FIG. 4. Directlybelow the second diffraction grating 48, the interference of the twopartial beams 50-1 and 52-1 occurs. The detection module records theinterference pattern generated. The contrast of the recordedinterference pattern contains the information about the spatialcoherence of the illumination radiation 21.

The coherence function of the illumination radiation 21 is determined bythe evaluation of the recorded interference pattern with an evaluationdevice. Preferably, a phase shifting technique is employed in order toincrease the accuracy of the evaluation. For this purpose, e.g. thesecond diffraction grating 48 can be rotated with respect to the firstdiffraction grating 46 for the spatial phase shift. Alternatively, thetwo gratings can be moved with respect to one another.

In accordance with a further embodiment the measurements are carried outwith different shear spacings, which are produced e.g. by the provisionof different grating constants or grating spacings of the diffractiongratings. Furthermore, the measurements can be effected for differentorientation directions of the diffraction gratings 46 and 48, e.g. for0° and 90°.

The spatial coherence function determined is thereupon converted intothe angularly resolved intensity distribution of the illuminationradiation 21 in the reticle plane 24 using the van cittert-zerniketheorem. The van cittert-zernike theorem is known to the person skilledin the art and describes the relationship between the extent of a lightsource and the spatial coherence of the radiation generated thereby.

FIG. 2 shows a further embodiment of the projection exposure apparatus10 according to the invention. The latter differs from the embodiment inaccordance with FIG. 1 in that the measurement system 40 is notintegrated into the edge of the reticle displacement stage 25. Rather,the optical module 42 and the detection module 44 of the measurementsystem 40 in accordance with FIG. 2 are arranged in different planes ofthe projection exposure apparatus 10 during the measurement process. Theoptical module 42 is arranged in the reticle plane 24, while thedetection module 44 is integrated into the wafer displacement stage 32in such a way that its detection area is arranged in the wafer plane 30and thus in a plane that is conjugate with respect to the reticle plane24.

In accordance with a variant according to FIG. 2, the optical module 42is designed in the form of a reticle and is loaded instead of a productreticle 22 by the reticle displacement stage 25 in order to carry outthe measurement process. The measurement is effected analogously to theprocedure described above. In accordance with a further variant of theembodiment according to FIG. 1, it is also possible to integrate theoptical module 42 in an edge region of the reticle displacement stage 25and to use for the measurement a detection module 44 integrated into thewafer displacement stage 32 in accordance with FIG. 2.

FIG. 3 illustrates a further embodiment of a projection exposureapparatus 10 according to the invention. The latter differs from theprojection exposure apparatus 10 in accordance with FIG. 1 in that themeasurement system 40 is not integrated directly into the reticledisplacement stage 25, but rather is inserted into the beam path of theillumination radiation 21 just above the reticle plane 24 using asuitable displacement device 45 in order to carry out the measurement.This has the advantage that for the measurement a reticle 22 arranged inthe reticle displacement stage 25 need not be removed and at the sametime a compact construction of the reticle displacement stage 25 ispossible. The arrangement of the measurement system 40 above the reticleplane 24 is effected so close to the reticle plane 24 that themeasurement system 40 is situated closer to the reticle plane 24 than tothe closest pupil plane 20 of the illumination system 16. Preferably,the distance from the closest pupil plane 20 is more than double themagnitude of the distance from the reticle plane 24.

In the embodiment in accordance with FIG. 3, the optical module 22 andthe detection module 44 are arranged close to one another, with theresult that the measurement system 40 is present with a compact design.According to an alternative variant, the entire measurement system 40 inaccordance with FIG. 3 can also be integrated into the waferdisplacement stage 32. In this case, it is necessary to remove thereticle 22 and the wafer 28 before the measurement.

FIG. 5 shows a further embodiment of the measurement system 40. Thisembodiment is likewise characterized by a compact design and can be usedin the projection exposure apparatus 10 for instance in the variants inaccordance with FIG. 1 and FIG. 3. The system 40 in accordance with FIG.5 comprises a focusing optical element, e.g. in the form of an opticallens element or a focusing mirror, as optical module 42. The detectionmodule 44 is designed as an areally resolving sensor and is arranged atthe focal point of the focusing optical element. In the embodiment inaccordance with FIG. 5, for measuring the entire illuminating field inthe reticle plane 24, the measurement system 40 is displacedsuccessively to different field points by a displacement device 49. Inthe case of the arrangement of the measurement system 40 in an edgeregion of the reticle displacement stage 25 analogously to FIG. 1, thereticle displacement stage 25 can serve as displacement device 49. Inthe case of the arrangement of the measurement system 40 above thereticle plane 24 in accordance with FIG. 3, the displacement device 49is designed as a separate system.

FIG. 6 illustrates a further embodiment of a measurement system 40according to the invention. It differs from the measurement system 40 inaccordance with FIG. 5 in that the optical module 42 is formed by atwo-dimensional arrangement of focusing optical elements 54. Thedetection module 44 is designed as an areal, spatially resolving sensorand comprises a CCD module, for example. Preferably, the extent of thearrangement of focusing optical elements 54 in the x-y plane extendsover the entire illuminated field in the reticle plane 24. It is thuspossible to measure the angular distribution of the illuminationradiation 22 in the entire illuminated field in parallel.

FIG. 7 shows a further embodiment of the detection module 44 inaccordance with FIG. 5. In accordance with this embodiment, thedetection module 44 is not embodied as an area sensor, but rathercomprises only a limited number of point sensors. Thus, for instance,two or four point sensors can be provided. As illustrated in FIG. 7, thedetection module 44 can be embodied as a four-quadrant diode, forinstance. Under (a) in FIG. 7, such a four-quadrant diode is illustratedfor the case in which the illumination radiation 21 is present ascoherent illumination, and for the case of quadrupole illumination under(b). Alternatively, it is also possible to capture the intensity withoptical fibers and to carry out detection outside the measurement system40.

The embodiment of the detection module 44 as illustrated in FIG. 7 makesit possible to detect important information about the illuminationradiation 21. This includes, for instance, in the case of (a)information about the energetic centering of the illuminationdistribution and in the case of (b) information with regard to thebalance of the individual poles. The embodiment of the detection module44 as an arrangement of point sensors has the advantage of lowcomplexity, which facilitates integration and accelerates themeasurement process.

FIG. 8 illustrates a further embodiment of a measurement system 40according to the invention. This measurement system, too, comprises anoptical module 42 and a detection module 44 in the form of an areallyresolving sensor. The optical module 42 comprises a pole selectiondevice 56 and a shadow casting element 61 in the form of a coarsegrating, comparable with gratings used in Moire methods. The poleselection device 56 comprises a first stop 58 having a cutout 59, and asecond stop 60, which is arranged below the first stop 58 in such a waythat it blocks one of the two poles 21-1 or 21-2 when a dipole-typeillumination distribution is radiated in. It is thus possible toseparately measure the radiation distribution of one of the two poles21-1 or 21-2.

The radiation of the illumination pole transmitted by the pole selectiondevice 56 thereupon passes through the shadow casting element and isregistered by the detection module 44. The divergence of the radiatedpole 21-1 is thereupon determined from the position of the transitionbetween a shaded region and an illuminated region on the detector areaof the detection module 44.

In accordance with one variant of the optical module 42 in accordancewith FIG. 8, a further shadow casting element is arranged in proximityto the detection area of the detection module 44. In this case, aone-dimensionally resolving diode can be used as detection module 44. Inaccordance with a further variant, the intensity distribution measuredby the measurement system 40 according to FIG. 8 is evaluated usingphase shifting methods. Here the stops 58 and 60 and/or the shadowcasting element 61 and/or a further shadow casting element arranged onthe detection area are moved or rotated with respect to one another.

The pole selection device 56 in accordance with FIG. 8 can also becombined with the optical module 42 in accordance with FIG. 4 if thepole selection device 56 is arranged above the two shearing gratings 46and 48.

The measurement system 40 according to the invention in one of theembodiments described above is particularly suitable for measuringillumination settings generated by illumination systems having amultimirror array. Such a multimirror array, also designated as MMA, isillustrated in FIG. 9 and designated by the reference sign 62. Itcomprises a two-dimensional array of micromirrrors 64, at least some ofwhich can be tilted at least with respect to one axis with tiltingdevices 66. It is thus possible to manipulate incoming partial beams 68individually with regard to their direction of propagation inreflection, such that the corresponding emerging partial beams 70acquire a direction of propagation that can be set individually in eachcase. Embodiments in which the illumination system of the projectionexposure apparatus comprises such a multimirror array are described forexample in WO 2009/080279 A1 and WO 2005/026843 A2.

FIGS. 10 to 12 illustrate a further aspect of the invention. This aspectrelates to the measurement of a beam divergence in an illuminationsystem of a projection exposure apparatus for microlithography at alocation in the beam path of the illumination system, which location isdesignated in greater detail below.

FIG. 10 illustrates an example of an illumination system 116 at whichthe beam divergence is measured with the method according to theinvention. Disposed upstream of the illumination system 116 is anirradiation source 112 for generating electromagnetic radiation, forexample having a wavelength in the UV wavelength range, such as, forinstance, 365 nm, 248 nm or 193 nm. The illumination system 116comprises a beam expanding optical unit 118, a beam angle redistributionmodule in the form of a first diffractive optical element 120, a beamstructuring element 123, a second diffractive optical element 128, aninput coupling optical unit 129, a rod 134, a field stop 136 and a REMAlens 138.

The electromagnetic radiation 114 in the form of an optical radiationbeam firstly passes through a beam expanding optical unit 118. Thelatter expands the beam cross section of the beam. The expanded beamsubsequently passes through the first diffractive optical element 120.The diffractive optical element 120 serves as a beam angleredistribution module and has the function of individually deflectingthe partial beams of the expanded beam which are parallel to one anotherupon impinging on the element 120, in accordance with a predefinedangular distribution. The function of the beam angle redistributionmodule therefore lies in generating a desired illumination angledistribution. Instead of the diffractive optical element 120 shown, thebeam angle redistribution module can e.g. also be formed with the aid ofa multimirror array, the individual mirrors of which are mounted in atiltable fashion. An illumination system comprising such a beam angleredistribution module is described for example in WO 2005/026843 A2.

The beam of the electromagnetic radiation 114 is thereupon transferred,with the illumination angle distribution impressed by the firstdiffractive optical element 120 into a downstream pupil plane by thebeam structuring module 123. This pupil plane, which is not illustratedin more specific detail, is situated in proximity to the seconddiffractive optical element 128. The beam structuring module 123comprises, for the further structuring of the radiation beam, a zoomsystem 125, schematically represented by a moveable lens element, and aso-called axicon, schematically represented by two optical elements. Bymoving the axicon elements apart, it is possible to set the inner sigmaof an illumination setting, or the boundaries of the cross section ofthe beam of an illumination setting. Secondly, it is possible to set theouter sigma of the illumination setting, or generally the outer boundaryof the beam cross section, by the movement of the zoom or of the lenselement illustrated schematically.

With a suitable design of the diffractive optical element 120 and asuitable choice of the position of the axicon elements and of the zoom,it is possible to generate any desired intensity distribution at theoutput of the beam structuring module 123 in proximity to the seconddiffractive optical element 128. A field angle distribution is impressedon this intensity distribution in the pupil plane by the seconddiffractive optical element 128, in order to obtain a desired fieldshape in a field plane, such as e.g. a rectangular field shape having anaspect ratio of 10:1. This field angle distribution of the beam in thepupil plane is transferred by the downstream input coupling optical unit129 into an illumination field 132 at the input of the rod 134.

In this case, the illumination field 132 at the input of the rod 134 issituated in a field plane of the illumination optical unit 116 and hasan illumination angle distribution having a maximum illumination anglevalue, which generally, but not necessarily, corresponds to thenumerical aperture of the preceding input coupling optical unit 129. Theillumination field 132 at the input of the rod 134 is transferred into afield 135 at the output of the rod 134. In this case, the maximumillumination angles in the field 135 of the rod output correspond tothose in the field 132 of the rod input. As a result of multiple totalinternal reflections at the walls of the rod 134, secondary lightsources having the field shape of the field 132 at the rod entrance asthe shape of each individual secondary light source arise at the rodexit in the exit pupils of the field points of the field 135. As aresult of this kaleidoscope effect of the rod 134, the field 132 ishomogenized with regard to the intensity distribution over the field,since as it were the light of many secondary light sources issuperimposed in the field 132.

The field stop 136 delimits the field 135 in the lateral extent thereofand provides for a sharp bright-dark transition of the field. Thedownstream REMA lens 138, as it is called, images the field 135 onto areticle 122 arranged in the reticle plane 124. In this case, thebright-dark edges of the field stop 136 are transferred sharply into thereticle plane 124.

The illumination system 116 comprises an exchanging device, which isillustrated with a double-headed arrow 171 in FIG. 10. The exchangingdevice 171 is configured for exchanging the first diffractive opticalelement 120 for a divergence amplification module according to theinvention in order to carry out the divergence measurement. In otherembodiments of illumination systems, not explained in greater detailhere, it may be necessary to exchange a plurality of optical elements inorder to carry out the divergence measurement. In accordance with oneembodiment, the divergence amplification module is configured as 4f lenselement array. As an example of such a 4f lens element array, amicrostructured plate 170 is described in greater detail below withreference to FIG. 11. A macroscopic telephoto lens is also considered asa further example of a 4f lens element array.

The microstructured plate 170 illustrated schematically in greaterdetail in FIG. 11 comprises two microlens element arrays 172 and 174spaced apart from one another. The two microlens element arrays 172 and174 are in each case embodied two-dimensionally, wherein the focallength f₁ of the first microlens element array 172 is greater than thefocal length f₂ of the second microlens element array 174. The microlenselement arrays 172 and 174 are opposite one another at the distanced=f1+f2. In order to ensure an optimum integration into the projectionexposure apparatus, the microstructured plate 170 is produced with thedimensions of the first diffractive optical element 120. When themicrostructured plate 170 is arranged at the location of the firstdiffractive optical element 120 in the beam path of the illuminationoptical unit 116, the plate 170 fulfils the function of a telescopeoptical unit and thus magnifies the divergence of the incoming beam ofthe electromagnetic radiation 114. The beam size changes upon passingthrough the plate 170 in accordance with m=f₁:f₁, while the divergenceis proportional to 1: m.

As a result of the dimensioning of f1>f2, as mentioned above, the beamdivergence can be magnified using the plate 170. This beam divergencemagnification makes it possible to carry out an angular distributionmeasurement for instance using a pupil measuring device already known tothe person skilled in the art from the prior art, the functioning ofsaid pupil measuring device being illustrated in FIG. 12. In accordancewith one embodiment according to the invention, the angular distributionmeasurement is carried out using one of the measurement systems 140according to the invention which are presented below and have alreadybeen described above. This can be effected using a measurement system 40which comprises the optical module 42 in accordance with FIG. 4 and adetection module in one of the arrangements in accordance with FIGS. 1to 3. That is to say that at least the optical module 42 in accordancewith FIG. 4 is arranged in the reticle plane 124 or just above thelatter. Furthermore, the angular distribution measurement can beeffected using one of the measurement systems 40 shown in FIG. 5 andFIG. 6 in an arrangement in accordance with FIG. 1 or FIG. 3. In thiscase, the measurement system as a whole is arranged in the reticle plane124 or just above the latter.

Furthermore, in principle, a measurement of the beam divergence at thelocation of the first diffractive optical element 120 can also beeffected, for example by the measurement system 40 that results from thecombination of the measurement module 42 in accordance with FIG. 4 witha detection module 44 or the measurement system in accordance with FIG.5 or FIG. 6 being inserted into the beam path of the illumination system116 in the place of the first diffractive optical element.

FIG. 12 schematically shows a projection exposure apparatus 110 havingthe radiation source 112 and the illumination system 116. The projectionexposure apparatus 110 furthermore comprises a projection lens 126 andan area-resolving detection module 144 integrated into a waferdisplacement stage. For pupil measurement, a special measurement reticle122 a is loaded into the reticle plane 124. The measurement reticle 122a has a multiplicity of punctiform test structures 176, which can beembodied either as so-called pinholes or else as opaque punctiformstructures.

If the detection module 144 is then arranged in a conjugate pupil planeat a position above or below the image plane 130 of the projection lens126, the intensity distribution in a pupil plane 178 of the projectionlens 126 is generated on the detection area 144 a of the detector module144. Said intensity distribution is also designated as “pupilogram” inthe general part of the description. The position of the detectionmodule 144 above the image plane 130 is also designated as intrafocalposition and is identified by the reference sign 180 in FIG. 12. Theposition below the image plane 130 is analogously designated asextrafocal position 182.

The intensity distribution measured by the detection module 144 when thelatter is arranged in the position 180 or 182 corresponds to the angulardistribution in the plane in which the first diffractive optical element120 is usually arranged. However, the measurement accuracy of the methoddescribed with reference to FIG. 12 does not suffice for the purposes ofdetermining the divergence of the electromagnetic radiation 114 at theoutput of the beam expanding optical unit 118. At the output of the beamexpanding optical unit 118, the beam divergence is typically 1 mrad,which corresponds approximately to a divergence of 20 msigma (FWHM) inthe case of a long zoom focal length. In the case of the pupilogrammeasurement using the arrangement from FIG. 12, 1 pixel of the detectormodule corresponds to approximately 10 msigma, i.e. changes indivergence of the relevant order of magnitude of approximately 0.1 mradare hardly measurable.

In order to solve this problem, as mentioned above, the microstructuredplate 170, instead of the diffractive optical element 120, is insertedinto the beam path directly at the output of the beam expanding opticalunit 118. By virtue of the beam expansion effected thereby, using thearrangement from FIG. 12 the beam divergence can now be measured with anaccuracy which suffices for carrying out the optimization of opticalproximity corrections on product reticles as described in the generalpart of the description.

The above description of the preferred embodiments has been given by wayof example. From the disclosure given, those skilled in the art will notonly understand the present invention and its attendant advantages, butwill also find apparent various changes and modifications to thestructures and methods disclosed. The applicant seeks, therefore, tocover all such changes and modifications as fall within the spirit andscope of the invention, as defined by the appended claims, andequivalents thereof.

LIST OF REFERENCE SIGNS

-   10 Projection exposure apparatus-   12 Radiation source-   14 Electromagnetic radiation-   16 Illumination system-   18 Optical axis-   20 Pupil plane-   21 Illumination radiation-   21-1 First pole-   21-2 Second pole-   22 Reticle-   24 Reticle plane-   25 Reticle displacement stage-   26 Projection lens-   28 Wafer-   30 Wafer plane-   32 Wafer displacement stage-   40 Measurement system-   42 Optical module-   44 Detection module-   45 Displacement device-   46 First diffraction grating-   48 Second diffraction grating-   49 Displacement device-   50 First partial beam-   50-1 Pivoted partial beam-   52 Second partial beam-   52-2 Pivoted partial beam-   54 Focusing optical element-   56 Pole selection device-   58 First stop-   59 Cutout-   60 Second stop-   61 Shadow casting element-   62 Multimirror array-   64 Micro mirror-   66 Tilting device-   68 Incoming partial beam-   70 Emerging partial beam-   110 Projection exposure apparatus-   112 Radiation source-   114 Electromagnetic radiation-   116 Illumination system-   118 Beam expanding optical unit-   120 First diffractive optical element-   121 Illumination radiation-   122 Reticle-   122 a Measurement reticle-   123 Beam structuring module-   124 Reticle plane-   125 Zoom system-   126 Projection lens-   127 Axicon-   128 Second diffractive optical element-   129 Input coupling optical unit-   130 Image plane-   132 Illumination field-   134 Rod-   135 Field-   136 Field stop-   138 REMA lens-   144 Detection module-   144 a Detection area-   170 Microstructured plate-   171 Exchanging device-   172 First microlens element array-   174 Second microlens element array-   176 Test structure-   178 Pupil plane-   180 Intrafocal position-   182 Extrafocal position

What is claimed is:
 1. A projection exposure apparatus formicrolithography comprising a radiation source for generating exposureradiation, and an illumination system disposed downstream of theradiation source and configured for radiating the exposure radiationinto a reticle plane of the projection exposure apparatus, wherein theillumination system comprises: a beam expanding optical unit forexpanding a beam cross section of the exposure radiation from theradiation source, a beam angle redistribution module for deflectingpartial beams of the exposure radiation from the beam expanding opticalunit, a divergence amplification module for amplifying a divergence ofthe beam from the beam expanding optical unit, and an exchanging devicefor exchanging at least one element of the beam angle redistributionmodule for the divergence amplification module.
 2. The projectionexposure apparatus according to claim 1, further comprising a projectionlens for imaging mask structures from the reticle plane onto a wafer, awafer stage for holding the wafer, and a system for measuring anillumination angle distribution in the reticle plane, wherein the systemcomprises a detector module arranged at the wafer stage and the detectormodule is arranged in a region of the conjugate pupil plane of theprojection lens during the measuring.
 3. The projection exposureapparatus according to claim 1, wherein the beam angle redistributionmodule comprises a diffractive optical element.
 4. The projectionexposure apparatus according to claim 1, wherein the divergenceamplification module comprises a microlens element array.
 5. Theprojection exposure apparatus according to claim 1, wherein thedivergence amplification module comprises two microlens element arrayshaving different focal lengths.
 6. A method for measuring an angularlyresolved intensity distribution in a reticle plane of a projectionexposure apparatus for microlithography comprising an illuminationsystem, which is configured for irradiating a reticle arranged in thereticle plane and has a first pupil plane, wherein all planes of theprojection exposure apparatus which are conjugate with respect to thefirst pupil plane are further pupil planes, and the reticle plane andall planes which are conjugate with respect to the reticle plane arefield planes, comprising: arranging an optical module in a beam path ofthe projection exposure apparatus above the reticle plane, arranging aspatially resolving detection module in a region of one of the fieldplanes such that the detection module is at a smaller distance from theone field plane than from a closest one of the pupil planes, radiatingelectromagnetic radiation onto the optical module with the illuminationsystem, and determining an angularly resolved intensity distribution ofthe radiated radiation from a signal recorded by the detection module.